CAMARILLO, Calif. (AP) — Investigators have ruled out arson as the cause of a huge Southern California wildfire that was 75 percent contained Sunday after burning through coastal mountains and threatening thousands of homes.

Continuing cool, moist air significantly reduced fire activity and helped firefighters build containment lines around the 44-square-mile fire at the western end of the Santa Monica Mountains.

Officials expected full containment late Monday.

Investigators said the cause of the fire was not considered suspicious. Instead, they believed it was started by a small, “undetermined roadside ignition of grass and debris” on the edge of U.S. 101 near Thousand Oaks, said Tom Piranio, a spokesman with the California Department of Forestry and Fire Protection.

The area near an uphill incline is considered a collection point for fuel and ignition sources, and it’s possible a piece of debris fell into the tinder-dry brush early Thursday and sparked the fire, Piranio said.

“The topography plus the hot, windy weather created a perfect storm for the fire to spread fast,” he said.

At its peak, the fire threatened some 4,000 homes as it moved through neighborhoods of Camarillo Springs and Thousand Oaks. It has caused damage to 15 homes.

The blaze is one of more than 680 wildfires in the state so far this year — about 200 more than average.

East of Los Angeles in Riverside County, a fire that burned 510 acres south of Banning was fully contained Sunday.

In Northern California, a fire that has blackened 11 square miles of wilderness in Tehama County was a threat to a pair of commercial properties near the community of Butte Meadows, according to Cal Fire.

Thunderstorms were expected to bring erratic winds but little rain to the area about 200 miles north of San Francisco.

Nearly 1,300 firefighters were on the lines and the blaze, which started Wednesday, was 40 percent contained.